The present invention relates to an isolator used for electrically separating and insulating between circuits and a modem using the isolator.
To protect extremely public network facilities and protect terminals, the communication field requires a high insulation quality for boundaries (hereinafter referred to as line interfaces) between networks and terminals and miniature communication transformers having a high insulation quality have been used.
However, as personal terminals have come into wide use and expanded recently, more miniature and light terminals are required as portable terminals, and a problem arises that the need for miniaturization is not sufficiently filled by improvement of the material and structure to be used for a transformer, and an application of the isolator has been examined.
In uses such as measurement and medical treatment, there is a case that it is necessary to insulate between the signal detection unit and the signal processing unit such as a sensor and a signal processing circuit and the isolator is known as an insulation separation means in such a case.
In such a case, although the signal voltage is about 100 mV, since a case that the commercial power supply comes in contact is assumed, there is a case that 100 V or a higher voltage is applied as a common mode noise voltage. In these respects, the isolator and line interface have a common problem from the viewpoint of high dielectric strength, miniaturization, and low cost.
The isolator is also the function itself of an insulating transformer, though there is a problem imposed that noise is mixed during transfer of a signal. For example, when a large common mode noise voltage is applied from the commercial power supply, a small signal transmission transformer may not transmit a signal and a transformer type isolator using a dedicated pulse transformer is used. The isolator using an insulating transformer is generally large in the mounting configuration and apt to be expensive.
To improve it, an insulation amplifier using a photo coupler combining a light emitting device and a photo detector has been designed. However, the photo coupler type insulation amplifier easily changes the characteristics due to temperature and to make it highly precise, improvements of the number and arrangement of light emission and detection diodes and circuits have been proposed but they are expensive. Users request further miniaturization.
However, if it is attempted to particularly realize a monolithic semiconductor, a semiconductor process of another material for light emission and detection is required in addition to the silicon semiconductor process, and by use of several kinds of manufacture processes, it is expected that such a monolithic semiconductor will be remarkably expensive, and it cannot be practically realized.
For the purpose of miniaturization, high reliability; and low cost, capacitive isolators have been developed. As a highly dielectric capacitor art as an individual part constituting an insulating barrier, a ceramic capacitor for power or surge protection is known and a circuit block for signal transmission using it is called a capacitive insulating amplifier or capacitive isolator and has been used since 1970s.
As a transmission system for transmitting a signal via a capacitive insulating barrier, the PWM system (called the pulse width modulation system or duty control system) is mainly used. This PWM art is known as a constitution art of an insulating barrier using an insulating transformer or photo coupler before it is used for this capacitive isolator.
In the capacitive isolator, furthermore, for the purpose of miniaturization, low cost, and high reliability, an insulating amplifier of a duty cycle modulation system is proposed using an insulating barrier which is a capacitor with a small capacity formed on a ceramic substrate and a floating comparator.
For further miniaturization, a proposal of reducing the capacity is made and an art of an insulating amplifier for changing a transfer wave form to a differential wave form using a small insulating barrier such as about 1 to 3 pF, reproducing an FM (frequency modulation) or PWM wave form from the differential wave form, and then demodulating is proposed.
With respect to application of a line interface of a modem and others, in U.S. Pat. No. 4,757,528, “Thermally coupled information transmission across electrical isolation boundaries” (hereinafter referred to as Patent 528) and ISSCC86 conference record THPM14.3 (hereinafter referred to as Announcement), Scott L. Falater (Harris Semiconductor) et al. disclose an idea of realization of a monolithic semiconductor using a capacitive insulating barrier.
Although it is not monolithic, three capacitive insulating barriers and a modem application circuit system of digital PWM signal transmission using them are proposed in Japanese Patent Application Laid-Open 7-307708.
For these circuits, a request of further miniaturization and lower cost will be presented in the future and when these prior arts are examined from this point of view, the following subjects and problems arise.
According to the art before Patent 528, an insulating barrier having highly dielectric performance, an input circuit receiving an input signal and generating a PWM waveform, and an output circuit reproducing and demodulating a PWM waveform are different parts and these parts are assembled and mounted so as to constitute one isolator. For example, a capacitive insulating barrier is structured on a ceramic substrate and at least two semiconductor chips are mounted on the same package so as to constitute an isolator. Namely, a constitution using many parts is provided.
In Patent 528 and Announcement, it is indicated that as an idea that a line interface which is an application circuit comprises a monolithic semiconductor, a capacitive insulating barrier and the PWM transmission system are used by the circuit schematic diagram which is a principle and explanation. The manufacturing method is that an isolator comprising a capacitive insulating barrier using the DI (dielectric separation) process and a PWM circuit is formed on a monolithic semiconductor and a sound band signal is transmitted by combining this isolator.
However, the disclosed art is an art regarding control of an insulating switch by a thermal pulse and it is not disclosed that an insulating barrier and circuit of what a kind of structure are structured on a monolithic semiconductor substrate by what a kind of method and as a result, how the structure operates and what an affect the structure produces.
Furthermore, in Japanese Patent Application Laid-Open 7-307708, although two insulating barriers are conventionally used for one transmission path, a circuit constitution for transmitting three signals by three capacitive insulating barriers is indicated but how the circuits operate so as to transmit signals is not indicated. Needless to say, a proposal for making these circuits including the insulating barriers monolithic is not made.